Video images are a more effective means of communication when compared to still images. Various electronic devices such as televisions and video record players that display video images have become very common household devices.
A television (TV), for example, operates by displaying video images in response to composite video signals received from a TV station or a video recorder attached to the television. A composite video signal comprises a luma signal and a chroma signal. The luma signal contains the luminance information, i.e., the grayscale or brightness details, pertaining to the video images. The chroma signal, on the other hand, contains chrominance information pertaining to the color details of the video images.
During encoding of the composite video signals for transmission purposes, the chroma signal is modulated into a subcarrier signal and is mixed with the luma signal to limit the transmission bandwidth of the composite video signals. Thus, the chroma and the luma signals may partially share the same range of frequencies. When received by the TV, the composite video signal is input to a video decoder, which separates the chroma and the luma signals and performs a series of signal processing operations to display the video images.
In general, the quality of separation of the chroma and luma signal of the composite video signals determines the quality of the video images displayed. In other words, to display high-quality video images, absolute separation of the chroma and the luma signals is required. However, this requirement is difficult to be fulfilled since some luma signals have frequencies that fall near the chroma subcarrier frequency and thus may not be separated by the video decoder.
Incomplete separation of the chroma and luma signals results in cross-luminance or cross-chrominance effects.
To prevent cross-luminance or cross-chrominance effects, video decoders employ sophisticated filters to enhance the separation of the chroma and the luma signals. Different types of filters, such as notch filters and comb filter, are used for that purpose. Comb filters are known to display a superior performance amongst the various other available filters.
Comb filters basically utilize two techniques for separation of the chroma and the luma signals: line combing and frame combing. Line combing is generally utilized to display those portions of the video images that undergo motion while frame combing is applied to the portions that are still. Depending on the detection of motion in a video image to be displayed, a switching operation, i.e., dynamic switching between a line filter and a frame filter of the video decoder, may be performed for every pixel of the image.